COS 16-5 - Precipitation seasonality mediates shrub-grass interactions

Abstract: In temperate drylands, grasses are dominant where most precipitation arrives during the warm season and soil water resources are shallow, while shrubs become increasingly important where cool season precipitation contributes to soil water at depth. In the western United States, big sagebrush (Artemisia tridentata Nutt.) ecosystems span broad gradients of precipitation amount and seasonality, with shrubs and grasses co-dominant under some conditions, and grasses sub-dominant to rare in others. Although plant-plant interactions may play a role in determining plant community structure in these ecosystems, the nature of shrub-grass interactions under different precipitation regimes remains relatively unexplored. Our objective was to characterize interactions between shrubs and herbaceous plants along gradients of precipitation amount and seasonality. We measured grasses and forbs in 15 pairs of shrub canopy and interspace microsites at 50 relatively undisturbed big sagebrush sites across the West. Mean annual precipitation (MAP) at sample locations ranged from 154 to 690 mm, and winter fraction of precipitation ranged from < 0.1 to 0.45. We estimated the outcome of shrub-grass interactions by comparing the size, height, cover, and density of herbaceous plants growing under and between shrubs using a relative interaction index (RII) that ranges from -1 (competitive) to +1 (facultative). RII is calculated using measures of plants from canopy and interspace microsites as follows: (Canopy value - Interspace value)/(Canopy value + interspace value). For perennial grass cover, RII was positively correlated (r = 0.51, p < 0.001) with winter fraction of precipitation, but not significantly correlated with MAP, and trends were similar for plant size and density. This suggests that precipitation seasonality may be more important than total moisture in determining shrub-grass interactions, which switch from competitive to facultative as conditions become more favorable for shrubs (higher fraction of winter precipitation). Although interactions between shrubs and other functional types (perennial forbs, annual forbs, and annual grasses) ranged from facultative to competitive across sites, RIIs were not significantly related to climate. Plant height tended to be greater under shrubs than in interspaces for all functional types under all conditions. Our results suggest that the relative importance of shallow and deep soil water may control the ability of shrubs and grasses to effectively partition soil water resources.